Corrosion and corrosion protection on steel ships

Steel rusts when exposed to water, oxygen and salt — and a ship's hull is exposed to all three parts all the time. Corrosion is therefore one of the enemies a ship fitter constantly deals with, whether it is the hull, piping systems or other steel structures on board. Fortunately there are well-tested methods to significantly slow down corrosion.

§Cathodic protection — the principle

Cathodic protection is a method to protect metal structures from corrosion in water, damp soil or concrete using electrochemistry. The basic principle is to make the steel hull the cathode in an electrochemical circuit so that another, more 'base' metal corrodes instead of the hull.

§Sacrificial anodes—zinc, aluminum, or magnesium

The most widespread method aboard ships is sacrificial anodes. Because the anode is less noble than the steel, it becomes the anode in the electrochemical element, while the steel structure becomes the cathode — and it is therefore the anode that corrodes, rather than the hull, hence the name 'sacrificial anode'. In saltwater, zinc or aluminium is typically used, because the high conductivity in the water means the necessary current can be achieved with a lower voltage difference between anode and hull; at lower conductivity, magnesium may be preferred. Sacrificial anodes are very common for cathodic protection of ships and marine installations.

• 01Zinc and aluminium anodes — widely used for ships and marine structures in salt water
• 02The anodes are mounted on the hull and must be inspected and replaced when worn out
• 03The wear grade of the anodes tells something about how well the protection works
• 04Missing or expired anodes significantly increase corrosion rate

§Applied current (ICCP) — the active method

Impressed Current Cathodic Protection (ICCP), in Danish called imposed current, protects metal against corrosion using an external power source. The current is automatically adjusted based on measurements from reference electrodes, so the metal is maintained at the protected potential. The technique is often used on large structures like ships, offshore platforms, pipelines and quay facilities where long-term durable corrosion protection is important — unlike sacrificial anodes, ICCP requires power supply and electronic control, but can provide more precise and long-lasting protection.

§Paint systems as the second line of defense

Beyond the electrochemical protection, marine paint systems are the first physical barrier between the steel and the corrosive environment. A good coating system typically consists of several layers — a primer, intermediate coat and topcoat — each with their function, from adhesion to the steel to resistance to wear and ultraviolet light. Damage to the paint, for example from a blow or grinding, should be repaired quickly because it is precisely here that corrosion typically takes hold first.

As a ship fitter it is your job to recognise signs of failing corrosion protection — corroded anodes flaking paint rust — and know when it is time to inspect or replace the protection before corrosion really takes hold of the structure.